Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS₂ Films

Abstract

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS2) films. To this end, we deposit a model heterostructure of thin amorphous MoS2 films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS2 into crystalline MoS2 domains, thereby emulating widely used elevated temperature MoS2 synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS2 films in situ and at the atomic scale. Our observations suggest that during MoS2 processing, various MoS2 polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS2 layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS2 crystallization can be influenced by a weak MoS2/graphene support epitaxy. Our atomic-scale in situ approach thereby visualizes multiple fundamental processes that underlie the varied MoS2 morphologies observed in previous ex situ growth and processing work. Our work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.

Epitaxial Growth of Wafer-Scale Molybdenum Disulfide/Graphene Heterostructures by Metal–Organic Vapor-Phase Epitaxy and Their Application in Photodetectors

“…The islands reshape into a thermodynamically stable configuration, which implies that the triangular shape is grown near the adsorption–desorption equilibrium condition. In contrast, the low diffusion energy barrier of the graphene substrate 32 , 33 allows the Mo and S adatoms to easily identify the energetically favorable site to restructure the MoS 2 domain 34 even at a low temperature of 400 °C. Because the graphene substrate maintains the equilibrium to preserve the triangular morphology, the growth of MoS 2 at a high temperature focuses on slowing down the growth rate and therefore enlarging the grain size.…”

Engineered 2D Transition Metal Dichalcogenides—A Vision of Viable Hydrogen Evolution Reaction Catalysis

“…Upon the chemical doping at the atomic‐level, the slight change of the catalyst may not be detectable by the frequently used “long‐term” (e.g., hours or days) polarization curves of the power samples and ex situ characterizations. Other in situ observations of the catalyst may need to be involved by integration with the conceptual device, such as the electrochemical scanning tunneling microscope (ECSTM), [ 189 ] the scanning transmission electron microscopy (STEM), [ 190 ] the tip‐enhanced Raman‐SPM/AFM (SPM: scanning Probe Microscopy) combined microscopy and beyond. [ 191 ]…”

Atomic‐Scale Fabrication of In‐Plane Heterojunctions of Few‐Layer MoS2 via In Situ Scanning Transmission Electron Microscopy

“…In addition to the first isolation of graphene layers, in situ TEM has been used for various graphene studies, including studied of the edge dynamics of graphene, the transformation of graphene to fullerene, and a high biasing experiment . Novel 2D‐TMDs, which are post‐graphene materials, are now mainstream materials in this field . Though several articles have reported the atomic‐scale structure of 2D‐TMDs and the dynamics of the defects and edges, most of these studies pertained to single layer materials .…”

“…However, beam‐emitted electrons which undergo high‐angle elastic scattering will then transfer an amount of kinetic energy and provide the energy for atomic displacement. This is why the electron beam can induce recrystallization and phase transition and threshold damage in 2D‐TMDs . On the other hand, one general viewpoint is that atoms migrate faster at high temperatures.…”

“…The ADF‐STEM images shown in Figure further investigate the structure with atomic resolution. By normalizing the image intensity of a single S atom and a single Mo atom in a 1H monolayer, where the contrast intensity is directly proportional to the atomic mass Z of Z 1.64 , we can identify the number of layers in terms of thickness . For MS‐MoS 2 , as shown in Figure a, the heterointerfaces of 3L/2L and 2L/1L were successfully created (Figure c,d).…”